On the Origin of Eddies Discovered during the POLYMODE Local Dynamics Experiment

Abstract

The intensive hydrographic survey conducted in 1978 as part of the POLYMODE Local Dynamics Experiment (LDE) revealed a large number of anomalous water parcels at various depths near 31°N, 70°W in the Sargasso Sea. From analyses of the mean water properties in the LDE survey, Lindstrom and Taft have prepared a census of discrete features that have statistically significant anomalies of salinity, oxygen or potential vorticity. The present analysis utilizes the climatological (annual mean) North Atlantic dataset compiled by Levitus to identify the large-scale distribution of salinity and oxygen on specific potential density surfaces. This technique has revealed distinct geographic origins for the majority of the 23 LDE features analyzed. Nine shallow features residing between 25.8 and 26.4 sigma-theta were found to have varied origins: two from the eastern Atlantic, three from the Gulf Stream, two from the central Sargasso Sea, and two were indeterminate with the available data. Four of six upper-thermocline features (26.5 to 26.9 sigma-theta) clearly originated in the eastern Atlantic, whereas the other two were derived from Sargasso Sea water. All five of the mid- and lower-thermocline features (27.0 to 27.5 sigma-theta) originated in the western Atlantic: three from the Antilles region, one from the Gulf Stream and one from the Sargasso Sea. The three subthermocline features residing between 27.7 and 27.8 sigma-theta were all derived from Labrador Sea water that can be found in the vicinity of the Grand Banks. The distinct origins of these features and their inferred trajectories contribute to our understanding of the general circulation within the North Atlantic, and suggest that discrete eddies may represent an important mechanism for the large-scale exchange of properties in the ocean.